Abstract

The purpose of this study was to investigate how the geometry of a fiber optic probe affects the transmission and reflection of light through the scleral eye wall. Two geometrical parameters of the fiber probe were investigated: the source-detector distance and the fiber protrusion, i.e. the length of the fiber extending from the flat surface of the fiber probe. For optimization of the fiber optic probe geometry, fluorescence stained choroidal tumor phantoms in ex vivo porcine eyes were measured with both diffuse reflectance- and laser-induced fluorescence spectroscopy. The strength of the fluorescence signal compared to the excitation signal was used as a measure for optimization. Intraocular pressure (IOP) and temperature were monitored to assess the impact of the probe on the eye. For visualizing any possible damage caused by the probe, the scleral surface was imaged with scanning electron microscopy after completion of the spectroscopic measurements. A source-detector distance of 5 mm with zero fiber protrusion was considered optimal in terms of spectroscopic contrast, however, a slight fiber protrusion of 0.5 mm is argued to be advantageous for clinical measurements. The study further indicates that transscleral spectroscopy can be safely performed in human eyes under in vivo conditions, without leading to an unacceptable IOP elevation, a significant rise in tissue temperature, or any visible damage to the scleral surface.

Figures (9)

Photograph of a cross-sectioned porcine eye. (a) Choroidal tumor phantom in the suprachoroidal space. Note that the phantom is in close contact with the surrounding tissues. (b) The crystalline lens in the anterior segment of the eye. (c) The optic nerve entering the posterior pole of the eye.

Optical setup used during the experiments. A fiber-coupled halogen lamp delivered a wide spectrum covering the visible and near-infrared spectral regions through a multimode (MM) fiber (600 μm core) to the eye. Light was collected with a second multi-mode fiber and coupled to a spectrometer for spectroscopy measurements. Sequentially, a 785 nm diode laser was used together with an 810-nm interference filter, which attenuated the excitation light to a level were both the fluorescence and the excitation signal could be measured simultaneously.

Schematic illustration of the experimental setup and the principle of transscleral diffuse optical spectroscopy. (a) Porcine eye in a gelatin-filled plastic container placed on an electronic scale. (b) Cross section of the probe and the eye. The optical fibers (for incident and detected light) are fixed by two plastic screws and centered on the scleral surface over the tumor phantom. The phantom (red) is located in the suprachoroidal space between the sclera (white) and the retina and retinal pigment epithelium (light blue and black). (c) Front and side view of the probe end. The fiber protrusion t, from the distal end of the probe, can be varied between 0, 0.5, 1.0 and 1.5 mm. The source-detector distance d equals 3, 4, 5 or 6 mm for the four different probes used. The diameter D of the probe itself is 10 mm.

Scanning electron microscopy image of the outer scleral surface from an eye that has been measured by a probe with 5 mm fiber source-detector distance and a fiber protrusion of 0.5 mm. The dashed circles indicate the areas where the optical fibers have indented the sclera. Note the plain surface and lack of any imprint from the fibers.

Tables (1)

Table 1 Measure of the fluorescence from the stained phantom in comparison to the transmitted excitation light, Γ, in 6 different porcine eyes with varying fiber source-detector distance and fiber protrusion. The ‘x’ indicates the source-detector distance or protrusion used in combination with the varied source-detector distance or protrusion.

Metrics

Table 1

Measure of the fluorescence from the stained phantom in comparison to the transmitted excitation light, Γ, in 6 different porcine eyes with varying fiber source-detector distance and fiber protrusion. The ‘x’ indicates the source-detector distance or protrusion used in combination with the varied source-detector distance or protrusion.